The invention relates to a method of manufacturing a security device, in particular incorporating an optically variable effect structure.
Optically variable effect structures such as holograms and diffraction gratings have been used widely over the last few years to impart security to documents of value such as banknotes, credit cards and the like. Conventionally, the structure is provided on a transfer foil and is then hot stamped from the transfer foil onto the final substrate. An early example of this approach is described in U.S. Pat. No. 4,728,377.
Hot stamp transfer techniques have worked well but involve a number of stages which it would be desirable to reduce. More recently, therefore, techniques such as in-situ polymerisation replication (ISPR) have been developed in which a polymer is cast or moulded against a holographic or other optically variable effect profile continuously while the polymer is held on a substrate, the profile then being retained by curing on or after removal from the profiled mould. Examples of this approach are described in U.S. Pat. Nos. 3,689,346, 4,758,296, 4,840,757, 4,933,120, 5,003,915, 5,085,514 and DE-A-4,132,476. In many of these examples, the optically variable effect structure is formed on a carrier for subsequent transfer although in some cases the structure is formed directly on the end product substrate.
Although these processes have been in existence for some time, little attention has been paid to the nature of the substrate on which the structure is provided.
In accordance with the present invention, a method of manufacturing a security device comprises providing a radiation curable material on a shrinkable substrate; imparting an optically variable effect structure on or into the radiation curable material; and curing the material so that it retains the optically variable effect structure.
We have realised that the known techniques of providing an optically variable effect structure directly on an end product substrate, for example ISPR, are much more flexible than has previously been realised and it is possible to use such a technique to place directly an optically variable effect structure onto a heat or chemically shrinkable substrate such as a shrink sleeve material. This leads to a number of advantages.
The process can be operated at high speed, for example up to 50 meters/minute or more while providing high fidelity, stable structures in the final product.
Some advantages of the invention, particularly with polypropylene substrates, are that it gives good impressions and release, is readily generated and replaced, gives good run lengths and also importantly does not absorb UV wavelengths as does PET if irradiating through it. This gives good use of the light input without being heated up or degrading.
A primary advantage of the present invention is that it enables a continuous in-line process to be utilised for its production in contrast to conventional hot stamping and other transfer processes.
Most conventional heat shrinkable materials can be used for the substrate, typically polymer films which have been bi-axially oriented. Some examples are given later.
Their surfaces will vary and different curing formulations will be needed as appropriate together with different surface treatments or primers as will be described below.
Important properties of the substrate material include:
Wetting ability to accept and retain optically variable effect structure after all curing and other processes have been completed.
Adhesion retention and ability to accept shrinkage stress and temperatures on heat shrink activation.
Must not attack, craze or prematurely degrade through the use of solvents, plasticisers, process conditions and the like.
The radiation curable material will typically be a resin which may typically be of two types:
a) Free radical cure resins which are unsaturated resins or monomers, prepolymers, oligomers etc. containing vinyl or acrylate unsaturation for example and which cross-link through use of a photo initiator activated by the radiation source employed e.g. UV.
b) Cationic Cure resins in which ring opening (eg epoxy types) is effected using photo initiators or catalysts which generate ionic entities under the radiation source employed e.g. UV. The ring opening is followed by intermolecular cross-linking.
An important aspect of these resins is that they can be cured at modest (less than 50xc2x0 C.) or even ambient temperature while operating at realistic production speed and therefore reduce the risk of damage to the structure by avoiding local overheating attack or stress. They can also be used as thin layers and provide efficient conversion of radiation energy to heat.
The radiation used to effect curing will typically be UV radiation but could comprise electron beam, visible, infra-red or higher wavelength radiation, depending upon the material, its absorbance and the process used.
The substrate material will typically be transparent so that the optically variable effect structure can be provided on a surface of the substrate which will not be externally exposed in use, while permitting the optically variable effect to be viewed through the substrate.
The structure may contain a hologram or diffraction grating and is preferably applied in the direction of least shrinkage of the substrate. This assists in preserving the form of the structure and optical replay after heat shrinking has been carried out. (The hologram or diffraction grating will preferably be viewable under white light illumination but could also be viewable under non-white light e.g. UV or IR.) The direction of least film shrinkage will be dependent on the manner in which the film or substrate has been manufactured. Shrinkage films are made by stretching the base material and orienting the molecules. The resultant products may then shrink on thermal or chemical exposure by 55-70% across the film and 5-7% in the linear direction. Depending on the article and the shrink required we would normally apply the images so that the replay would be in the linear direction i.e. least shrink effects.
Typically, the structure will be compressed on heat shrinkage by about 5% and thus must be resilient so that it will retain its integrity. Stress from heat and shrinkage must not destroy or significantly reduce the image replay or cause loss of adhesion, mechanical stress, cracking, etc.
With some substrates such as PVC, retention of the integrity of the structure will occur without further action being required. In other cases, however, such as on PET substrates, a pre-treatment may be required to achieve bonding, colouring or other effects or mechanical stress relief layers. This priming or pre-treatment may involve corona treatment with or without a thin layer coating before ISPR or a suitable thin layer directly applied to the substrate material. Corona discharge treatment will clean the surface and improve its wetting and bonding capacity to a primer layer or to the ISPR (or radiation curable) layer often without the use of a separate thin primer layer.
Before or after the provision of the cured optically variable effect structure, the substrate may be provided with other indicia by, for example, being printed, particularly on its surface opposite to that on which the radiation curable material is provided. The indicia may comprise a decorative finish, manufacturer""s details and the like. In addition, the printing may define windows through which the optically variable effect can be viewed.
The substrate itself may be transparent or tinted and the method may further comprise providing a high reflective layer, such as a metallisation or a high refractive index material, over the profile. This promotes the replay of the optically variable effect but will not always be necessary particularly if subtle effects are required.
If a metallisation is provided, this may be partially demetalised to achieve a patterning effect while a further protective lacquer could be applied over the optically variable effect generating structure either before or after curing.
In a further option, an adhesive layer could be is provided over the optically variable effect generating structure which enables the structure to be adhered to an article in a similar manner to that which is described in more detail in WO-A-96/27178. This is particularly useful where the finished product is to be used in the form of a shrink sleeve or the like.
For products where high security is not critical, the optically variable effect generating structure may be provided on the surface of the substrate which will be external in the finished product. In this case, the substrate could be opaque or have low transparency.